V(X1)=1 and V(X2)=4
The constraint since E(X1)=z=E(X2) is a+b=1, where z should be lower case, not upper case.
So use calc 1 to minimize given a+b=1, by inserting one of these two variables,a,b.
You could use LaGrange, but cal 1 is easier.
I get a=4/5, b=1/5 giving me 20/25=.8 as the answer.